The present invention relates to a forced-air cooling apparatus of a steam turbine in a high temperature state just after an operation shutdown of the steam turbine and, more particularly, to a forced-air cooling apparatus of a steam turbine having a double casing structure capable of safely and quickly cooling the turbine.
Generally, a steam turbine system is composed, as shown in FIG. 10, of a high-pressure turbine 1, an intermediate-pressure turbine 2 and a low-pressure turbine 3, and the high-pressure turbine 1 and the intermediate-pressure turbine 2 may be connected together through a common shaft or through two shafts.
Referring to FIG. 10, reference character A represents a boiler, and a main steam generated by the boiler A is supplied into the high-pressure turbine 1 through a main steam inlet portion of the turbine 1 by way of a main steam line 4, a main steam stop valve 5 and a main steam governing or control valve 6. The main steam, after the working in the high-pressure turbine 1, is then exhausted through a steam outlet portion into a high-pressure outlet line 7. The steam exhausted into the high-pressure outlet line 7 is heated by a reheater B and the reheated steam is then supplied into the intermediate-pressure turbine 2 through a reheated steam inlet portion thereof by way of a combined reheater valve 8. The steam, after the working in the intermediate-pressure turbine 2, is exhausted into a cross-over line 9 through a steam outlet portion of the turbine 2 and then supplied into a low-pressure turbine 3. The steam, after working in the turbine 3, is then passed to a condenser 10. Further, a degree of vacuum condition in the condenser 10 is maintained by a vacuum pump 11.
The high-pressure turbine 1 in FIG. 10 has a double casing structure such as shown in FIG. 11 as one example. Referring to FIG. 11, the steam from the boiler A is introduced into the high-pressure turbine 1 in a manner such that the steam enters through the control valve 6 into either of the main steam inlet portions 12a of a high-pressure outer casing 12 and then into a high-pressure inner casing 13. The steam entering the high-pressure inner casing 13 then passes through nozzles 13 fixed to the inner casing 13 and turbine blades 14a fixed to a rotor 14, as shown by arrows, in a steam flow passage and imparts a rotating force to the high-pressure rotor 14. During this operation, the pressure and the temperature of the steam lower and the steam then flows into the reheater B from a main steam outlet portion 12b through the high-pressure turbine outlet line 7.
In a case where the steam turbine system of the structure described above is periodically inspected or dismantled because of any failure or maintenance, the steam turbine system must be shutdown in its operation and the highly heated portions of the steam turbine then must be cooled to a point where dismantling can begin, thus being inconvenient and troublesome. For this purpose another apparatus for cooling the turbine system is required.
The temperature of the supplied steam is about 300.degree. C. which is relatively low, so that the cooling thereof can be done for a relatively short period by natural cooling the low-pressure turbine 3 after the operation shutdown thereof, thus no specific cooling apparatus being required.
On the other hand, the high-pressure turbine 1 and the intermediate-pressure turbine 2, into which the main steam from the reheater B reheated in temperature to about 500-600.degree. C. and the reheated steam are supplied, respectively, are under highly heated when these turbines 1 and 2 are shutdown in operations. Particularly, in the high-pressure turbine 1 in highly heated, the wall of the inner casing 13 is made thick in order to withstand the high-pressure steam. Accordingly, it takes a long cooling period to lower its temperature to a temperature suitable for the dismantling thereof through the natural cooling of the turbine. For this reason, the inspection or maintenance of the high-pressure turbine will have to be done after a relatively long period natural cooling of the turbine and the operation of the turbine system must be shutdown for a considerable long period, thus being inconvenient for the power supply, for example.
In view of the above fact, in a conventional technique, the shortening of the cooling period for the high-pressure turbine has been attempted by drawing air through a safety valve disposed in the outlet line of the high-pressure turbine and discharging the air into the condenser from the main steam line. (For example, see Accelerated Cooling of High-Output Turbines, Brown Boveri Rev. Vol. 63, No. 2, pp. 141-147, 1976; Forced Air Cooling of Steam Turbines, Convention On Steam Plant Operation, Conference Publication 12, pp. 199-205, 1973; Zwangsabkuehlung yon Turbinen der 500-MW-Bloecke durch Ansaugen van Luft, Energietechnik Vol. 34, No. 7, pp. 241-245, 1984).
However, such conventional forced-air cooling apparatus for the turbine system has the following drawbacks.
First, according to the conventional technique, as disclosed in the Japanese Patent Laid-Open Publication Nos. 56-32014, 56-162212 or Japanese Patent Publication No. 3-4723, there is provided a system in which air for cooling is charged through the high-pressure outlet portion at relatively low temperature during the steady running period of the turbine system and the air is then discharged through the main steam inlet portion. However, in such system, the cooling air passes through a turbine steam passage from a reverse direction with respect to a normal steam flow direction in turning operation, so that a large volume of air necessary for the cooling is not passed and, hence, it is difficult to shorten the period for cooling the turbine. In the conventional technique described above, the reason why the cooling air is charged through the high-pressure discharging portion and the air is discharged through the steam inlet portion is considered to suppress thermal distortion or thermal stress of the turbine which may occur at the period of forced-air cooling operation. However, this will be disregarded for the following reason.
Namely, thermal distortion or thermal stress of the extent causing fatigue to elements or members of the turbine will never occur by cooling, for about one day by forced-air cooling the high-pressure turbine under the high-temperature shutdown of about 500-600.degree. C. during the turbine operation period. This will be easily assumed from the fact that thermal distortion or thermal stress occurring at the time the steam turbine under the completely cooled operation starts to be driven with an operation starting period of about 8 to 16 hours to increase its temperature to about 500-600.degree. C. is within an allowable range.
Accordingly, it is not necessary to have a structure, as disclosed in the Japanese Patent Publication mentioned above to charge the external air through the high-pressure outlet portion by means of an air drawing apparatus connected to the main steam inlet portion, and in such structure, since a large volume of air necessary for sufficiently cooling the turbine cannot be passed, it takes a long period to cool the turbine to a temperature suitable for dismantling the turbine.
Secondarily, in the conventional technique such as disclosed in the Japanese Patent Publication mentioned above, a rotor is cooled faster than the cooling of a stationary portion such as an inner casing, which causes a differential expansion between the rotor and the stationary portion, which may cause a problem such that the rotor now in the turning operation comes in contact with a nozzle. Namely, since the air for cooling is discharged externally through the inside of the inner casing, the rotor having a surface area larger than that of the inner casing and a weight less than that thereof and blades mounted to the rotor are subjected to large cooling effect and, hence, the rotor and its blades are cooled faster than the inner casing. Because of this reason, the thermal expansion of a movable portion such as rotor is lowered faster than that of a stationary portion such as inner casing, thus causing the differential expansion therebetween.
As is generally known, the rotor and the nozzle of the turbine are so arranged in design that both are opposed in position with an axial, i.e. longitudinal direction of the rotor, with small clearance therebetween to the extent that both prevent to contact each other even from internal temperature change at a load varying period during a normal operation period. Accordingly, at the forced-air cooling period, if the rotor is cooled faster than the inner casing, the differential expansion between the rotor and the nozzle is caused, which may result in the rotor now in the turning operation contacting the stationary nozzle, which may cause a possibility of damaging the nozzle.
Furthermore, the high-pressure turbine having an inner casing in which a nozzle is provided is constructed such that a small radial clearance between the nozzle and the outer tips of the blades mounted to the rotor and a small radial clearance between a rotor shaft and a labyrinth packing provided for the inner periphery of the nozzle have proper distances so as to prevent to come in contact with each other during the steady operation period of the turbine. Because of this reason, there is caused a temperature difference between upper and lower halves of the casings, further causing a difference in their thermal expansion and, hence, deforming of the casing such as a round-shouldered shape, which may come in contact between the rotating rotor shaft and the stationary labyrinth packing or between the outer peripheries, such as its radial and axial directions of the blades and the nozzle.